Thyroid cancer is classified based on four tissue types: papillary cancer derived from follicular cells, follicular cancer, anaplastic carcinoma, and encephaloid carcinoma derived from parafollicular cells. Among thyroid cancers, anaplastic thyroid cancer (ATC) grows very rapidly, so that approximately 80% of anaplastic carcinoma patients die within 1 year even if treated (Passler, C., et al., Langenbecks. Arch. Surg. 384, 284-293, 1999; Voutilainen, P. E., et al., World. J. Surg. 23, 975-978, 1999). Therefore, treatment of anaplastic thyroid cancer is difficult. It is thus important to thoroughly treat differentiated thyroid cancer before anaplastic transformation. Accordingly, understanding of the development mechanism of anaplastic thyroid cancer and selection of differentiated thyroid cancer cases in which anaplastic transformation is likely to take place are required. Based on studies conducted to date, it is considered that gene alteration takes place successively like a chain reaction during the processes of cell differentiation and proliferation, resulting in canceration. However, the gene alteration that actually induces anaplastic thyroid cancer remains unknown. Therefore, there are currently no methods for detecting anaplastic thyroid cancer. Nor are there any methods for examining the malignancy of anaplastic thyroid cancer.